The present invention involves a system and method for fabricating a prosthetic limb socket, and more particularly to a system and method for insuring a wrinkle free thermoplastic prosthetic limb socket, having a consistent wall thickness, by determining the precise dimensions of an extended portion to be added to the proximate end of the positive cast (positive socket mold) for molding the prosthetic limb socket.
A prosthesis is often used to replace an amputated portion of a limb and to help restore the amputee's ability to use that limb. A prosthesis for a lower extremity amputation will often include an artificial foot connected to an upright assembly which is in turn connected to a custom fitted socket assembly. If the amputation is an above-the-knee ("AK"), the upright assembly will often commonly include an artificial knee joint.
The socket assemblies typically include an outer socket fabricated from a hard thermoplastic material. These outer sockets are typically created by heating a thermoplastic preform cone, stretching the heated cone over a positive cast of the amputee's residual limb and then vacuum forming the cone in place over the positive cast.
Presently there are several types and sizes of the thermoplastic preform cones. The types of material available for the preform cones include a clear thermoplastic material, a polypropylene polymer material, a polypropylene homopolymer material, and a flexible thermoplastic material. The available sizes for these preform cones typically range from a 16 cm base diameter to a 25 cm base diameter. The size of the preform cone to be selected typically is based upon measured geometrical dimensions of the positive cast of the residual limb.
To create the outer socket from the preform cone, the preform cone is then heated and stretched over the positive cast of the residual limb. These heating and stretching steps are commonly performed on a Socket Manufacturing Unit ("SMU"). A typical SMU will include a vertically translatable platform for mounting the preform cone thereto. The positive cast of the patient's residual limb will be placed onto a suction seat, which is in vertical alignment with and below the platform. The platform is first lifted such that the preform cone enters into a heating chamber at the top of the SMU, where it is heated for a sufficient amount of time to become soft and pliable. The heated preform cone is then lowered by the platform to stretch over the positive cast and suction seat. Next, the heated preform cone is sealed over the positive cast and the circumference of the suction seat, and a vacuum pump creates a vacuum between the heated preform cone and the modified positive cast, causing the preform cone to be intimately molded over the positive cast. Once the thermoplastic material has cooled and sufficiently solidified, the positive cast is extracted from the thermoplastic material and excess thermoplastic material is removed from the molded socket, leaving a finished thermoplastic socket component; and once this socket component has been fabricated, the remaining portions of the patient's prosthetic limb can be assembled.
To attain a desired thickness of the finished socket, the preform cone must typically be stretched a certain distance beyond the proximate (or proximal) end of the positive cast (i.e., the thickness of the socket will be linearly dependent upon the amount that the preform cone has been stretched along or beyond the positive cast); and typically, the fabricator must estimate the amount that the preform cone must be stretched beyond the positive cast of the residual limb to attain the desired thickness. To extend the length of the positive cast, and in turn, to control the thickness of the finished socket, the fabricator will typically attach or stack objects (such as wooden blocks) to the proximate end of the positive cast, thus lengthening the positive mold.
As discussed above, the size of the object to attach to the proximate end of the positive cast is estimated by the fabricator based primarily upon the fabricator's experience with the process. A disadvantage with this estimation is that it is very difficult to obtain a desirable thickness of the prosthetic limb socket, often requiring the fabrication process to be repeated.
Another problem with the present socket manufacturing processes is that the preform cone has a mouth at its proximate end with a circumference which is typically significantly larger than the circumference of the positive cast (or proximate built-up portions) at the proximate end of the positive cast (or proximate built-up portions). Accordingly, when the preform cone is heated and suction fit over the positive cast and any proximate built-up portions, the extra plastic material of the preform cone material at the proximate end of the positive cast will sometimes wrinkle and form creases along the built-up portions. These creases and wrinkles will many times extend all the way up to the portion of the finished socket which covers the positive cast of the residual limb. This portion of the finished socket is often the portion to remain after the waste portions have been discarded; and thus, creases or wrinkles extending into this often cause fracture points or cause `pinching` or other discomfort to the wearer. Accordingly, creases or wrinkles extending into this `remaining` portion of the socket, for all intents and purposes, ruins the socket, requiring the socket to be refabricated.
Accordingly a need exists for a system and method which will allow the prosthetic limb socket fabricators to consistently control the thickness of the prosthetic limb sockets which are created by molding a preform cone over a positive cast of the patient's residual limb. There is also a need for a way to substantially eliminate the formation of creases and wrinkles in the finished version of the thermoplastic socket.